The interaction between lipids and ammoniacal nitrogen mitigates inhibition in mesophilic anaerobic digestion

Ammoniacal nitrogen and long chain fatty acids (LCFA) are common inhibitors of the anaerobic digestion process. However, the interaction between these inhibitors has received little attention. Understanding the interaction between these inhibitors is important to optimise the operation of anaerobic digesters treating slaughterhouse waste or using fat, oil and grease (FOG) as co-substrate among others. To study the interaction between ammoniacal nitrogen and LCFA inhibition, 20 different conditions were trialled in mesophilic batch tests. Experimental conditions included 5 mixtures between slaughterhouse wastewater and LCFA (100:0, 75:25, 50:50, 20:80, 0:100 on a VS basis), each one tested at 4 different ammoniacal nitrogen concentrations (0, 1, 3, 6 gN_added·L^-1). Experimental and modelling results showed that ammoniacal nitrogen inhibition was less severe in LCFA-rich mixtures, indicating that LCFA mitigated ammoniacal nitrogen inhibition to a certain extent. However, the positive interaction between inhibitors did not only depend on the LCFA concentration. A protective LCFA coat that limited the diffusion of free ammonia into the cell and/or provided a localised lower pH in the vicinity of the microbial cell could explain the experimental results. However, ammoniacal nitrogen and LCFA inhibition comprise up to 6 different but interrelated inhibitors (i.e. NH₃, NH₄⁺, LCFA, VFA, H₂ and pH) and therefore the specific mechanism could not be elucidated. Nonetheless, these results suggest that LCFA do not exacerbate TAN-related inhibition and that LCFA-rich substrates can be utilised as co-substrates in mesophilic N-rich digesters.